Non-motoring > Screw thread for round metal electrical box Miscellaneous
Thread Author: Dulwich Estate Replies: 18
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New Mon 21 Oct 13 17:07 
 Screw thread for round metal electrical box - Dulwich Estate
I have been volunteered to fix up a few (not heavy) ceiling light fittings.

The ceiling lighting circuit is in metal round conduits terminating at the lighting position with a round metal box. I am familiar with square metal boxes for mains outlet plates but not used the round type in a ceiling before. The conduits were installed in the late 1960s and the wiring is fine and relatively new.

I know that both the square and round boxes have pressed out lugs with threaded holes to accept fixing screws. My question (because I am a long away from the site and don't have a round metal box to hand) is...

Is the thread diameter and type the same as in the square metal boxes used for outlet sockets ? I have plenty of such screws which is good; I might need to shorten some - that's all though. But if they don't fit I'd lose half a day finding the right ones.
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New Mon 21 Oct 13 17:28 
 Screw thread for round metal electrical box - crocks
I'll share what little I know.

Modern square metal boxes for sockets and switches use M3.5 threads.

I have removed old 1950s/60s sockets which are held on with similar sized screws but which are different.
A quick Google suggests these may be 2BA.
So it may be worth tracking some of these down.

Hth.
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New Mon 21 Oct 13 18:16 
 Screw thread for round metal electrical box - crocks
>> I have removed old 1950s/60s sockets which are held on with similar sized screws but
>> which are different.
>> A quick Google suggests these may be 2BA.

I have now rummaged and found some of the old pattresses and screws.

By measuring with my imperial calipers and referring to here...
en.wikipedia.org/wiki/British_Association_screw_threads
....it looks like the old size for sockets was 4BA.

However the link sherlock has provided suggests 2BA for conduit boxes.
Last edited by: Crocks on Mon 21 Oct 13 at 18:17
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New Mon 21 Oct 13 17:32 
 Screw thread for round metal electrical box - sherlock47
Standard metal and plastic back boxes use M3.5 however I have come across some non standard metal boxes (I think used in ex local authority housing) that use a different thread and the ones I have seen use brass pan headed screws. I have tried googling but so far with no results - but I remember being told to make sur that you keep the screws 'they are like rocking horse poo'.

So you are right to be asking the question!

crocks has beaten me to it - but I think that 2BA would seem about correct!
Last edited by: VxFan on Mon 21 Oct 13 at 21:36
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New Mon 21 Oct 13 22:45 
 Screw thread for round metal electrical box - MD
Short, Brass (slotted) Pan head screws kept here in the van. 30 or 40mm I think. If you only needed a few then mail me via the Mod folk.
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New Mon 21 Oct 13 17:35 
 Screw thread for round metal electrical box - henry k
Check Google for BESA boxes and BESA box screws?

I was beaten x2. IIRC 2BA screws
Last edited by: henry k on Mon 21 Oct 13 at 17:36
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New Mon 21 Oct 13 17:37 
 Screw thread for round metal electrical box - sherlock47
www.diynot.com/forums/viewtopic.php?t=189801

all explained!
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New Mon 21 Oct 13 17:42 
 Screw thread for round metal electrical box - crocks
And just to confuse matters, modern conduit boxes have M4 threads.
www.screwfix.com/p/20mm-2-way-galvanised-conduit-box/46004
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New Mon 21 Oct 13 17:40 
 Screw thread for round metal electrical box - henry k
>>I have been volunteered to fix up a few (not heavy) ceiling light fittings.

Pendant light fittings (roses) used to have the same centres for fitting such boxes.
Worth checking if possible.
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New Mon 21 Oct 13 17:47 
 Screw thread for round metal electrical box - henry k
I have a baccy tin full of various 2BA screws of various lengths /types that, if you wish, will get you out of trouble in the short term.

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New Mon 21 Oct 13 18:27 
 Screw thread for round metal electrical box - Roger.
"I have been volunteered" -- don't you just love the implication? :-)
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New Mon 21 Oct 13 22:01 
 Screw thread for round metal electrical box - Ted

When I've come across a wall box with a damaged thread, I've found a self tapper will do the job. Size 6, I think. Woodscrews and self tappers seem to be the same nowadays from Selco, where I buy them.

Ted
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New Mon 21 Oct 13 23:14 
 Screw thread for round metal electrical box - Dulwich Estate
I am overwhelmed with the response and kind offers of a few 2BA screws to tide me over.

As luck would have it or some might say it's careful planning on my side, I have a few days before I need to do the job so won't impose upon the kindness of the forum.

eBay was my friend, as ever, and I'm expecting 10 no. of the lovely screws on Wednesday.

Thanks all.

I have another question now - it's about gas ovens.
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New Tue 22 Oct 13 07:31 
 Screw thread for round metal electrical box - sherlock47
Woodscrews and self tappers are not rec as a solution as they invariably have a very sharp point. Cable penetration is easy if the box is crammed full of cables. But if they are blind holes I suppose it does not matter:)
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New Tue 22 Oct 13 08:17 
 Screw thread for round metal electrical box - Cliff Pope
Take a selection with you, plus a thread tapper for the largest likely size.
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New Tue 22 Oct 13 09:13 
 Screw thread for round metal electrical box - henry k
>> Woodscrews and self tappers are not rec as a solution as they invariably have a
>> very sharp point. Cable penetration is easy if the box is crammed full of cables.
>> But if they are blind holes I suppose it does not matter:)
>>
Never seen a BESA box with blind holes.
I am amazed at the bodgers in this world.
Not difficult to find what is the correct screw. BA screws were the norm 50+ years ago when I started in electronics :-)
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New Tue 22 Oct 13 10:07 
 Screw thread for round metal electrical box - VxFan
>> Woodscrews and self tappers are not rec as a solution as they invariably have a
>> very sharp point.

Not all self tappers have a sharp point at the end. Most, if not all the ones in my storage bin in the garage are flat ended and the hole you drill is only marginally smaller than the self tapper.
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New Tue 29 Oct 13 18:06 
 Screw thread for round metal electrical box - Dulwich Estate
Thanks to the brains on here I got myself 10 brass 2BA screws (from eBay).

I fitted 4 ceiling lights today and the 2BA screws were an absolute necessity - naturally they fitted perfectly and did a good job of securely fixing the fittings.

Thanks again.

I got 1.25 inch long screws expecting I might need to cut some down in length, but even where the supporting bar ended up flush with the ceiling I still managed to screw in the entire 1.25 inches with no cutting - they were mighty deep boxes (useful for poking up the choc block connectors).
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New Tue 29 Oct 13 19:51 
 Screw thread for round metal electrical box - henry k
Thanks for the end results.
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